höfuðin — their heads

[1] skóku höfuðin ‘shook their heads’:
An allusion to Matt. XXVII.39: praetereuntes
autem blasphemabant eum moventes capita sua ‘and they that
passed by blasphemed him, wagging their heads’. The poet probably also had in
mind Lam. II.15 and Ps. CIX.25, which are liturgically associated with the
Passion.